Positive corona stabilization apparatus for an electrostatic voltage generator



May 25, 1965 J R005 3,185,915

POSITIVE CORONA STAB ILIZATION APPARATUS FOR AN ELECTROSTATIC VOLTAGE GENERATOR Filed June 1, 1962 2 Sheets-Sheet 1 May 25, 1965 J. POSIT 00 NA STAB Filed June 1, 1962 ROOS IZATION APPARA FOR AN ECT TATIC TAGE GENERAT 2 Sheets-Sheet 2 3O 29 I5KV 20 /8 POWER SUPPLY F 3 22 T0 D/FFERENT/AL AMPLIFIER l6 United States Patent POSITIVE CORONA STABILIZATION APPARATUS AN ELECTROSTATIC VOLTAGE GENERA- Jan Roos, Arlington, Mass, assignor to High Voltage Engineering Corporation, Burlington, Mass., at corporation of Massachusetts Filed June 1, 1962, Ser. No. 199,445 9 Claims. (Cl. 322-2) This invention relates to electrostatic high voltage generators and more particularly to negative high voltage terminal machines and to apparatus for stabilizing the voltage thereof.

Electrostatic generators of the type comprehended by this invention are subject to considerable terminal voltage variation. This is due in part to surface conditions of the charging belt and to local corona conditions on the terminal and down the support members. Since the voltage at which such a generator operates is a function of the current taken up the charging belt and that lost by all paths to ground, there have been various voltage stabilization arrangements developed that operate through a controlled current leak to ground. It is also possible to govern the voltage of an electrostatic generator through control of the charging belt current. Because of the slow response associated with charging belt current control, however, it is not very satisfactory and is used chiefly for slow drift compensation. Perhaps the most effective solution to date, whereby fine, relatively fast control of current lost from the high voltage terminal is achieved, is that of inserting a corona point through the generator housing in the vicinity of the high voltage terminal and controlling the corona current flowing therethrough by means of a servo arrangement. In such a system, the corona current is controlled either by adjusting the distance between the corona point and the electrode or by changing the corona point potential. The latter method is preferable because of the simplicity of the electronic circuit involved. It is only necessary to insulate the corona point from ground, connect it to the plate of a grounded cathode triode, and control the corona current by biasing the grid. The control signal with which the grid is biased is obtained from, and is responsive to, changes in the ion beam generated by the electrostatic generator.

The above described electrostatic generator voltage control and stabilization system has proved to be particularly effective when used in conjunction with positive terminal machines. However, because of the difiiculty in obtaining sufiiciently large positive point to negative plane corona at high pressures, this approach has never found utility in negative terminal machines. Corona points of the type described above, when used in a negative terminal machine, exhibit extensive sparking. The erratic nature of such sparking, being incompatible with the constant current flow necessary for effective voltage stabilization, dictates against the use of conventional corona point control in negative terminal generators. Furthermore, the very high voltage electrostatic generators now being developed utilize higher density gases and higher pressures to ensure proper insulation within the housing structure. In such an environment it is extremely difiicult to obtain positive corona breakdown.

Therefore, because of these and other limitations, corona stabilization in negative terminal electrostatic generators has not heretofore been feasible. Currently, voltage stabilization in these machines is accomplished by means of the more complex generating voltmeter apparatus.

Accordingly, it is a principal object of this invention 3,185,915 Patented May 25, 1965 to provide, in a negative terminal electrostatic voltage generator, new and improved corona stabilization apparatus.

It is another object of this invention to provide, in an electrostatic generator, positive point to negative plane corona voltage stabilization apparatus that is not sub ject to sparking.

it is still another object of this invention to provide, in an electrostatic generator, positive point to negative plane corona voltage stabilization apparatus that will operate effectively in a high pressure, high density gaseous atmosphere.

These, together with other objects and features of this invention will become more readily apparent from the following detailed description of one illustrative embodiment thereof taken in conjunction with the accompanying drawings wherein:

FIGURE 1 illustrates an arrangement of the various components of this invention and their relationship to an electrostatic generator;

FIGURE 2 illustrates the novel positive point corona head assembly comprehended by this invention; and,

FIGURE 3 is a sectional view of FIGURE 2 taken at Although this invention comprehends the broader aspects of electrostatic generator voltage stabilization including the complete system whereby such stabilization is accomplished, it is pointed out that the particular composition and geometry of the corona points employed are of singular importance. Corona points used for stabilizing positive machines are commonly of stainless steel and have a point tip radius of about .002 inch. It is essential to the present invention that the corona point tip radius be at least an order of magnitude less than this, and preferably in the .0003 to .00003 inch range. Such metals as platinum, tantalum, stainless steel, and molybdenum have been found to produce operable devices with platinum proving the most effective. While the diameter of the corona point beyond the tip is determined largely by the number of points used and by the amount of current per point for any given parameters, points having diameters in the .030 to .060 inch range with point tapers of from 6 to 1 to 20 to l are eliective for most applications.

Referring now to FIGURES 2 and 3, there is illustrated thereby a corona head assembly 17 of the type comprehended by this invention which employs a plurality of such corona points. An aluminum shield member 24, having apertures 25 through which corona points 26 protrude, is maintained at some discrete potential by power supply 19. Said power supply 19 is loaded by triode valve 21, the grid of which is biased by a control signal from amplifier 16 as hereinafter described. Bushing member 23, of insulating material such as nylon or the like, is adapted to position shield member 24 and corona point holding member 29 in spaced, electrically isolated relationship with the generator housing. Each corona point 26 has a threaded distal end 27 whereby it may be screwed into holding member 29 to provide proper protrusion of the point through shield 24. It is desirable that a plurality of points be used, preferably an array of between ten and fifty. While the exact number of points depends upon the particular parameters in any given case, they should be spaced apart by approximately .4 inch and be in sufiicient quantity to ensure a load of less than ten micro-amperes per point. The protrusion of points 26 from shield 24 is restricted to one-quarter inch or less to allow corona head 17 to withstand large electric field gradients and to establish a more effective shield bias. Rubber gasket 30 is contiguous to holding member 29 and acts as a lock washer to the corona points screwed therethrough. The corona points are connected to ground through said holding member 29 and ammeter 18.

The complete positive corona voltage stabilizing system of this invention is illustrated together with an electrostatic voltage generator in FIGURE 1 to which reference is made in the following detailed description thereof. Electrostatic voltage generator 5 of the Van de Graaif type includes high voltage terminal 6, voltage gradient insulating structure 8, and belt charging system '7. Positive ions generated in ion source are accelerated as ion beam lfi by the high negative potential of terminal 6. The ion beam is deflected as it passes through electrostatic analyzer l]. and is utilized indirectly through interception with target 14 as shown, or directly by replacing target 14 with a window. Members l3, 13 are insulated from ground and from each other and are positioned as shown to establish a slit therebetween so as to allow the passage therethrough of ion beam it). When operating at the desired voltage, generator 5 accelerates the ions of ion beam 10 by an amount such that they are deflected by electrostatic analyzer 11 through an angle that causes them to pass between members l3, 13'. A high terminal voltage produces faster ions which are deflected less by electrostatic analyzer ill resulting in the interception of said ions by member 13. Conversely, a low terminal voltage produces slower ions which, being deflected more, intercept member 13. Interception of the ion beam by either of members 13 or 13 establishes a potential that is a measure of the ion current striking it. The signal obtained therefrom is amplified by differential pro-amplifier 15 and differential amplifier is and then applied as a bias to the grid of triode 21. Power supply 19 acting through load resistor 20 establishes a potential of up to 8 kv. on shield 24 of corona head assembly 17. The signal applied to the grid of triode 21 is adapted to change the positive bias voltage on said shield 24 such that at zero bias corona will be a maximum. The corona current, then, as it discharges through corona points 26 to ground is controlled by a signal responsive to the ion beam deflection. That is, it the ion beam velocity or voltage changes a small amount from that for which the voltage of electrostatic analyzer 11 is set, more of the beam will strike one of members 13, 13 than the other and a difference voltage will appear at differential pro-amplifier 15. This voltage is t. .en used to bias the grid of trlode 21. This comprises a servo system in which the voltage of electrostatic analyzer 11 is the standard and potentials developed on members 13, 13 are error signals that operate the generator voltage control system.

There has thus been disclosed means whereby the objects of this invention are accomplished and although specific apparatus has been referred to in the foregoing description, it is not intended that the invention be limited thereto, said apparatus being by way of illustration only. Numerous modifications and arrangements may be devised by those skilled in the art without departing from the incentive concepts taught herein. Therefore, the true scope of this invention is defined and limited by the following claims only.

I claim:

1. In combination with an electrostatic generator having a negative high voltage terminal, voltage stabilization apparatus comprising a plurality of grounded metal corona points in near proximity to said negative high voltage terminal, a metal corona point shield member having apertures therein, said corona points being electrically isolated from said shield member and arranged to protrude through said apertures, means for establishing a potential on said shield member, means for detecting on said negative high voltage terminal deviations from normal operating potential, means for providing a control signal responsive to said detected deviations, and means for varying the potential on said shield member in response to said control signal.

2. In an electrostatic voltage generator, means for establishing negative terminal to positive point corona discharge comprising an array of pointed metal mem bers, a metal shield member having apertures therein, the pointed ends of said pointed metal members being arranged to protrude through said apertures in proximate relationship to said negative terminal, said pointed metal members being electrically insulated from said shield member and having the distal ends thereof connected to a common electrical ground, and means for establishing an electrical potential on said shield memher.

3. Voltage stabilization apparatus as defined in claim 1 wherein said metal corona points are fabricated of pl atlnum and have a point tip radius of less than .0003 inch.

4. Voltage stabilization apparatus as defined in claim 3 wherein said metal corona points protrude through said apertures less than one quarter of an inch.

5. Voltage stabilization apparatus as defined in claim 4 wherein said plurality of corona points comprises a quantity thereof sufiiclent to limit the corona current flowing therethrough to less than ten microamperes per point.

6. Means for establishing negative terminal to positive point corona discharge as defined in claim 2 wherein said pointed metal members are fabricated of platinum and have a point tip radius of less than .0003 inch.

7. Means for establishing negative terminals to positive point corona discharge as defined in claim 2 wherein said pointed metal members are fabricated of platinum and have a point tip radius of substantially .0001 inch.

8. Means for establishing negative terminal to positive point corona discharge as defined in claim 7 wherein the pointed ends of said pointed metal members protrude through said aperture less than one-quarter of an inch.

9. Means for establishing negative terminal to positive point corona discharge as defined in claim 8 wherein said array of pointed metal members comprises a quantity thereof sufiicient to limit the corona current flowing therethrough to less than ten microamperes per point.

References tilted by the Examiner UNITED STATES PATENTS 2,548,452 4/51 Turner 322-2 MILTON O. HIRSHFIELD, Primary Examiner. 

2. IN AN ELECTROSTATIC VOLTAGE GENERATOR, MEANS FOR ESTABLISHING NEGATIVE TERMINAL TO POSITIVE POINT CORONA DISCHARGE COMPRISING AN ARRAY OF POINTED METAL MEMBERS, A METAL SHIELD MEMBER HAVING A APERTURES THEREIN, THE POINTED ENDS OF SAID POINTED METAL MEMBERS BEING ARRANGED TO PROTRUDE THROUGH SAID APERTURES IN PROXIMATER RELATIONSHIP TO SAID NEGATIVE TERMINAL, SAID POINTED METAL MEMBERS BEING ELECTRICALLY INSULATED FROM SAID SHIELD MEMBER AND HAVING THE DISTAL ENDS THEREOF CONNECTED TO A COMMON ELECTRICAL GROUND, AND MEANS FOR ESTABLISHING AN ELECTRICAL POTENTIAL ON SAID SHIELD MEMBER. 